Six months on the space station leaves astronauts slightly younger than everyone else
Orbiting Earth at about 7,700 metres per second, an astronaut who spends six months aboard the International Space Station comes home roughly 5 milliseconds younger than people who stayed on the ground. That tiny gap is time dilation, the relativity effect that satellite navigation systems such as GPS and Galileo must account for.
Clocks moving relative to an observer are measured to run slow, and the faster the motion, the bigger the lag. Several physicists anticipated the effect: Joseph Larmor in 1897 applied it to orbiting electrons and Emil Cohn in 1904 tied it to clocks. In 1905 Albert Einstein showed that it reflects the nature of time itself, and in 1907 Hermann Minkowski introduced the idea of proper time. Differences in gravity produce a separate version of the effect.
A simple thought experiment explains why. Picture a clock made of two mirrors with a pulse of light bouncing between them. To someone riding with it, the light goes straight up and down. To an observer watching it pass, the light traces a longer diagonal path. Since light has the same speed for everyone, the only way to reconcile the two views is for the moving clock's ticks to stretch out. Every other kind of clock must slow by the same amount.
Oddly, the effect is mutual: each of two observers in relative motion measures the other's clock lagging, much as two people far apart each see the other as small. That symmetry sets up the twin paradox, where a space-travelling twin nonetheless returns younger. Fast particles confirm it. Muons created by cosmic rays and moving at 98 per cent of light speed survive about five times longer than the 2.197 microseconds measured for slow ones in the lab.
In principle, extreme speed could carry travellers into the future, with a year aboard matching ten on Earth, and a steady 1 g acceleration could cross the known universe within a human lifetime. So far the record gains are modest: the cosmonauts Sergei Krikalev and Sergey Avdeev each drifted about 20 milliseconds behind Earth.
Source: Time dilation
